One of the most frequently used applications for the voltage sensor is in the remote monitoring of supply voltages in circuits whose proper operation is critical. Numerous examples can be found onboard space satellites and space research vehicles, the most notable being the NASA space shuttles. Their complex control circuitry must be supervised continuously from ground-level monitoring stations for the purpose of interpreting their performance. Circuitry malfunctions can be analysed and corrective command signals radioed back to the vehicle to prevent further problems from developing.
Space exploration satellites are another example of using voltage-monitoring sensing circuits. The satellite's power supply voltages are continuously monitored (there most likely being more than one voltage per supply) and the signal-converted information sent to the satellite's transmitter for transmission back to earth. All of the voltage sensors being used onboard the spacecraft are similar to the VCO sensors described earlier.
Figure 6-11 shows another example of an application of a voltage sensor. This is an example that can be used in the electrical power industry for the monitoring of line voltages along a power-line system. The line voltage is read by a VCO voltage sensor in which the incoming line voltage is inductively coupled through a transformer and then converted to a dc voltage by means of rectification. The converter then changes the voltage to audio oscillations for remote processing and interpretation. These monitoring stations can be located at periodic intervals along power-line routes for troubleshooting line conditions. The voltage sensors' audio output signals are transmitted by small low-powered transmitters located at each tower to passing service vehicles for "in-motion" line-voltage supervision from these vehicles.
Figure 6-11 Voltage-sensing transmitter for monitoring line voltages on a high- voltage transmission line.
A common problem associated with commercial radio and television transmission towers is the following. During electrical storms very high voltages can often develop at the antenna. These voltages are either static voltages created during the passage of overhead, highly charged storm clouds, or high-voltage spikes induced into the antenna as a result of nearby lightning strikes. In either case, voltage-level sensors can be installed on the antenna so that when a certain high voltage value is attained, the outgoing radio transmission is, automatically, interrupted momentarily. The antenna is shorted to ground until the antenna's electrical charge is harmlessly bled off. Figure 6-12 shows such an installation. When the voltage value at the antenna falls below the switching value, radio transmissions are continued. This type of voltage-level switch performs well when a certain amount of hysteresis is used. That is, the lowering voltage value that reactivates the switch can be at a lower value than the original rising voltage-level switching value. This prevents switch "chattering" from taking place in case the voltage on the antenna varies slightly on either side of the voltage sensor's preset switching voltage value.
Figure 6-12 Voltage sensor placed on a radio tower for detecting high static voltages. These are but a very few examples of voltage-sensing applications presently being used. Voltage sensing, especially static voltage sensing, is a relatively new sensing application, and more and more applications are being developed by industry.
Review Questions
6.1. Explain the difference between electrostatic voltage and non- electrostatic voltage. How do you think the difference affects the design of the voltage sensors that are used for detecting each?
6-2. Explain how the VCO can be used in a voltage detection circuit. 6-3. Show how an operation amplifier design can be used as a voltage-
level detection device. How does this design differ from a VCO? 6-4. Explain the difference between "sinking" an output signal or
"sourcing" such a signal. Draw schematics to illustrate your explanation.
6-5. Give a practical example of how a voltage sensor may be used at a remote site or installation to monitor a critical supply voltage.
6-6. Explain how a voltage detection circuit may be modified to monitor current usage in a particular circuit or system.
Problems
6-7. Calculate the potential difference between the old position and the new position of a charge resulting from moving a charge of 7.8C that required expending 17J of work to make the move.
6-8. Determine the energy that was expended in moving a charge of 37C over a potential difference of 10V.
6-9. If a change of 0.12V ac is required to produce measurable change in a voltage-sensing transducer's output, what is the transducer's responsiveness? The voltage change measurement was made at a voltage of 25.0V ac.
6-10. Determine the resolution for a digital voltage-sensing circuit having a 50V span whose output is capable of handling 8-bit data words. 6-11. A designer wants to design a voltage sensor that can detect voltages
as small as 50 mV out of a range of 0 to 100V. How many data lines will be necessary to handle the resolution desired?
References
Berlin, Howard M. The 555 Timer Applications Sourcebook, Indianapolis, IN: Howard W. Sams, 1979.
Buchsbaum, Walter H. Encyclopedia of Integrated Circuits, Englewood Cliffs, NJ: Prentice Hall, 1981.
DeMaw, Douglas ARRL Electronics Data Book, Newington, CT: American Radio Relay League, 1976.
Sessions, Kendall W. Master Handbook of 1001 Practical Electronic Circuits, Blue Ridge Summit, FA: TAB Books, 1975.
Chapter 7 - Variable Resistance Sensors
Chapter Objectives
1. To review the relationship between voltage and resistance in order to understand the characteristics of the potentiometer.
2. To understand the behavior of the linear and rotary displacement potentiometers.
3. To study the strain gage and its characteristics.
7-1 - Introduction
Of all the sensing devices available for transducing, the potentiometer device is perhaps the easiest to understand, the least expensive to construct, and the simplest to install. Its basis of operation lies within the conversion of a resistance change to a change in voltage or current: namely, the application of Ohm's law. However, another type of variable-resistance device, the strain gage, is perhaps not quite as straightforward as the potentiometer in operation, as we will find out in the discussions below. However, to understand either device, an understanding of Ohm's law and the concept of resistivity is certainly helpful. A review of how voltage divider networks operate will also contribute to an understanding of these devices.